Pretreatment of aluminum surfaces

ABSTRACT

A process for preparing the surface of aluminum products to receive colored coatings comprises a substantially pollution free, non-chromate pretreatment and aluminum conversion coating process. The process provides improved coating quality, increased process stability, increased throughput and extended operating life of the processing baths by controlling the electrical resistance of the deionized water used for washing the aluminum product being processed to at least about 50 k ohms and controlling the manner of washing between each step of the process.

The invention is directed to a process for preparing the surface ofaluminum to receive colored coatings. More particularly it is directedto a substantially pollution free, non-chromate pretreatment andaluminum conversion coating process which provides improved coatingquality, increased process stability and extended operating life of theprocessing baths.

BACKGROUND

It is well known that it is particularly difficult to apply coloredfinishes to aluminum surfaces because of the oxide coating whichnaturally forms on the aluminum surfaces soon after it is formed.Accordingly, aluminum or aluminum alloy surfaces are routinely treatedby applying an intermediate corrosion resistant conversion coating tothe surface. A broad range of subsequent coatings can then be readilyapplied to the conversation coating to produce an acceptable, blemishfree new surface. A common technique is to clean the aluminum surfaceand then apply a acid based hexavalent chromium composition to thatclean surface. The chromium conversion coatings have been readilyaccepted because they are generally corrosion resistant and provideexcellent retention of subsequent coatings. However, because of theextreme toxicity of chromium compounds and ecological problems resultingfrom waste disposal from aluminum treatment plants, the industry hasreplaced chromium conversion coating with chromium free systems.

A significant advance has been the use of potassium permanganate basedconversion coating systems, such as developed by SanChem. Representativeof such systems is the use of SanChem CC3400, a KMnO₄ based compoundwhich is chromium, cyanide and fluoride free. A typical SanChemtreatment composition comprises immersion of the aluminum product in a10% solution of SanChem CC3400+1.7 oz activator @ 135-140° F. for 1.5-2min to produce a gold color on the aluminum; longer time provides athicker coating with a darker yellow color. A preliminary step iscleaning the surface prior to the permanganate dips. If an alkalinecleaner is used it is usually followed by a deoxidizer or a mild, nitricacid based solution. Generally speaking the SanChem process comprisesdegreasing, an alkaline wash at 160° F. (such as SanChem 500,—a mildlyalkaline phosphate cleaner), deoxidizing at 125° F. using SanChem 1000which is a nitric acid based deoxidizer free of fluorides, chromates andheavy metals, forming an oxide film by immersion in DI Water @ 210° F.and a three step sealing process (SanChem 2000+SanChem 3000+SanChem4000).

SanChem has several patents directed to permanganate conversion coatingsof which the following are representative:

U.S. Pat. No. 4,711,667 describes degreasing the aluminum surface usingmineral spirits followed by an alkaline wash (NaOH, alkaline NaNO₃, HF,Na₂CO₃ or borax). This is followed by a water wash and then immersion ina bath of KMnO₄ with borax, sodium benzoate or sodium carbonate,typically for 1 minute at 155° F., followed by a water rinse. Allsolutions were preferably silicate free.

U.S. Pat. No. 4,988,396 is directed to aluminum alloys with 1% Cu. Afterthe surface is degreased it is deoxidized using a 10% nitric acidsolution (85° C., for 20 min.) followed by a deionized (DI) water rinseand immersion in 195°-212° F. DI water for 5 minutes to form a boehmitelayer followed by the permanganate treatment.

U.S. Pat. No. 5,437,740 has several examples for anodizing aluminum andconversion coating of aluminum castings. Room temperature DI waterwashes after several of the treatment stages are utilized. Deoxidizingis performed using 70% HNO₃, 2.4% HF, 27.5% water.

U.S. Pat. No. 5,707,465 uses a permanganate solution containinghexavalent chromium. The aluminum material is degreased (using anorganic solvent and a non-ionic detergent), rinsed with DI water,deoxidized using 10% HNO₃ at 70° F. for 1 minute and then rinsed with DIwater. It was then treated with various different permanganatesolutions.

U.S. Pat. No. 6,087,017 is directed to a corrosion resistant waxpolyester film for aluminum. However, the pretreatment process isrelevant. The aluminum panel is cleaned using a mild alkaline cleaner at150-160° F. for 3 minute, rinsed in DI water, deoxidized in 10% HNO₃ and3% sodium bromate at 120° F. for 5 minutes followed by a DI water rinse.This can then be followed by the application of the permanganateconversion coating at 150° F. for 1 minute.

U.S. Pat. No. 4,883,541 discusses the prior use of acidic deoxidizers incombination with HF. The claimed invention is then directed to acidicdeoxidizers, particularly 10% HNO₃ with 3% sodium bromate or iodate.

U.S. Pat. No. 5,192,374 is directed to a treatment following thedeoxidation step. The initial cleaning steps use an alkaline cleaner(Chemidize 740) at 71° C. for 3 min. followed by a rinse for 1 min in DIwater, deoxidizing at 30° C. using 10% HNO₃+3% sodium bromate, rinsingwith DI water for 1 min. and then immersion in boiling DI water for 5minutes.

U.S. Pat. No. 5,417,819 is directed to compositions for desmuttingaluminum surfaces comprising the use of 10-100% nitric acid and 15gr/liter of a fluoride, the balance being water with the possibleaddition of H₂SO₄ and/or H₃PO₄. The preferred source of fluoride ion isammonium bifluoride. The surface is first treated with an alkalinecleaner solution (A31K) at 140° F. It is then immediately rinsed withwater, preferably DI water. The surface is then subjected toelectro-brightening followed by a DI water rinse. Desmutting is thenperformed for 0.5-2 min @ 60-110° F. followed by a water rinse.

U.S. Pat. No. 6,123,782 is directed to a process for forming a coatedaluminum product comprising cleaning the surface using a nonchromated,nonsilicated alkaline cleaner followed by a hot water rinse, deoxidizingat up to 120° F. using a nonchromated deoxidizer such as SanChem 1000(10% nitric acid, 3% sodium bromate) and then immersion in boilingdeionized water for 5-10 minutes.

Prior available processes still suffer from several recurring problemswhich are not addressed by a mere optimization of the process variablesor use of alternative compositions. Initial operation of the processesset forth in each of the above references gives generally acceptableresults. However, after a short period of continuous. operation thequality of the surface produced, or subsequently applied surface,becomes unacceptable due to streaks, non-uniform appearance andsubsequent fading. The treatment baths have to be dumped frequently andreplaced with fresh baths in order to maintain acceptable coatingproperties. Prior techniques and/or solutions appear to include theexistence of unrecognized and unaddressed variables which contribute toa rapid decrease in product quality. Improving the process quality isnot a mere matter of optimizing the operating conditions. Applicant hasdiscovered that there is a necessity to scrupulously process, reprocessand filter the DI water to remove interfering ions which may be in thefeed and rinse water feed streams, a need to limit the time andtemperature of exposure of the aluminum to DI water and the importanceof subsequent use of both spray and immersion treatments. Pietschmann,J. and Jehn, Hermann (Powder Coatings, Pre-Treatment and Quality Controlof Aluminum for Architectural Applications, (Galvanotechnik, D-88348Saulgau, 91, (2000) Nr. 9)) addressed the various steps of theconversion coating process, namely degreasing, pickling/etching, aciddipping if alkaline pickling is used, conversion coating and drying. Theauthors states that:

-   -   “Between the individual steps, sufficient rinsing is necessary.        Rinsing is a diffusion controlled procedure. Not only the amount        of water, its purity and the temperature are decisive, but also        the duration of rinsing is important. The type of rinsing        technique, dipping or spraying, also influences the result”        However, the criticality of the washing steps, scrupulous        reprocessing of the water and the use of DI water at ambient        temperatures was not appreciated.

DETAILED DISCUSSION

A preferred process, incorporating features of the invention, comprises:

-   -   a) a cleaning step using a commercially available alkaline        composition in the manner and under conditions generally        suggested by the manufacturer,    -   b) use of 10% or greater HNO₃ as a deoxidizer with a 3% fluoride        solution (KF preferred) at room temperature, and    -   c) application of a SanChem conversion coating (SanChem 3400)        under conditions and in a manner substantially the same as        recommended by the manufacturer.

The improvement comprises the use of extensive DI water washes betweeneach step of the process. These water washes include a combination ofspray and immersion washes that appear to be critical to decontaminatingthe treated surface between each step to eliminate trace amounts ofprior applied materials. The DI water is preferably continuouslyreprocessed and filtered to remove undesirable ions and maintain theresistively within defined limits. Care is also taken to not use DIwater at elevated temperatures so as not to react with the surface (i.e.oxidize the surface) to form boehmite.

More specifically a process incorporating features of the applicant'sinvention comprises processing through a series of wash tanks asfollows:

-   -   1) The aluminum part is immersed (agitation optional but        preferred) in a first water bath containing a non-silicated,        non-caustic, alkaline cleaner (such as US Specialty Color Corp        Specialty 740), under conditions recommended by the supplier,        namely 6-8 oz of the cleaner/gal at 140-165° F. (preferably 155°        F.) for 3-7 min.    -   2) The rinse is followed by a 20-30 sec atomized spray rinse        using DI water. The aluminum part is then immersed in DI water        at ambient temperature for 45-60 sec, agitation optional. The        rinse is followed by a 20-30 sec atomized spray rinse using DI        water.    -   3) The aluminum part is immersed in DI water/Nitric acid (30% by        Vol)/KF1 (6-8 oz/gal) at ambient temperature for 15-60 sec. The        rinse is followed by a 20-30 sec atomized spray rinse using DI        water which drains into that dip tank.    -   4) The piece is then exposed to a 45-60 sec atomized spray rinse        using ambient DI water, a 45-60 sec immersion in ambient DI        water and a 20-30 sec atomized spray rinse using DI water.    -   5) The aluminum piece is then immersed in DI water/SanChem        CC3400 (10% by vol)+SanChem CC3400 Activator (1.7 oz/gal) at        140-165° F. (preferably 155° F.) for 2.5-4 min. Agitation        optional. The appearance of the product is light yellow.    -   6) The immersion is followed by 20-30 sec atomized spray rinse        using DI water, a 30-45 sec immersion in DI water and then a        20-30 sec atomized spray rinse using DI water.

It is preferred that all process equipment and contact surfaces (tanks,racks, baskets, hangers, etc.) are non-conductive, non-reactive materialsuch as polypropylene, polyethylene or Teflon coated surfaces so thatnone of the baths or wash solutions can leach any contaminants from theprocessing equipment. All of the DI water feed to the system as well asthe DI water spray and dip tank contents used in each wash stage maybegin with city and/or well water which is filtered through a initialwater purification system consisting of a) 0.5 micron particulatefilter, b) a D.I mixed-bed canister and c) a 0.5 micron particulate postfiltration at a rate of 10. gals/min upon demand. Once the DI water hasbeen processed through the initial water purification system and entersthe aluminum treatment system it is preferably collected andcontinuously filtered serially through a) a 0.5 micron particulatefilter, b) a carbon filtration canister, c) a cation bed, d) two anionbeds, e) a DI mixed bed canister and f) a final 0.5 micron particulatefilter at a rate of from about 3 to about 10 gal/min to maintain a waterquality of at least about 50 k ohms-cm. A typical DI immersion tankcontains 320 gal of DI water so that the content is recycledapproximately at least every 1.75 hours. Alternatively, a typicalconversion coating system could treat the rinse water simply as a wastesteam. Using a three stage cascading method of rinsing, the impuritiesfrom the cleanest rinse overflow to the medium purity rinse and fromthat into the dirtiest rinse. The dirtiest rinse is treated by adjustingthe pH and ORP but is discharged to the sewer on a daily basis. In anyevent, the processed DI water entering each washing stage is maintainedat a resistance of at least about 50 k ohms and at ambient temperatureto minimize oxidation which can occur when clean aluminum surfaces arecontacted with DI water at elevated temperatures.

Besides controlling the quality of the DI water used throughout thesystem, the chemical makeup of the water feed to the system andsubjected to deionization can also have an influence on the quality ofthe conversion coated product. A typical analysis of the feed waterentering the system is given in Tables 1 and 2. Table 3 lists theanalysis of that feed water returned to the system following filteringand RO treatment. Silica, an ion known to present aluminum conversioncoating processing problems, while present in the feed water, has beeneliminated by the filter and deionization process, and as such the watertreatment process contributes significantly to the production of qualitycoatings.

It has been found that operation of a process incorporating features ofthe invention results in significantly increased quantities ofacceptable finished product which is free of streaking, spotting,peeling and discoloration. Additionally, the life of the varioussolution baths (the time before the quality of the treated product isunacceptable) is increased to about 480 hours of production, resultingin a significant increase in process throughput and a commensuratereduction in the down time for cleanup and bath replacement as well as areduction in the cost for replacement bath solutions. Still further, thefrequency of discharge of bath contents, which contain EPA regulatedpollutants, into the surrounding environment is significantly reduced.TABLE 1 FEED WATER ANALYSIS Turbidity (Method 180.1) 0.1 NTU Turbidityafter N.M filtration Conductivity (Method 151.0 MMHOS/CM Fst TDS by143.7 Color (Method 2120C) 0.0 Color after N.M Acidification pH(Method150.1) 7.7 Tannins N.D. (Concentrations reported as mg/L (PPM) unlessotherwise reported) CATIONS (Method 200.7) ANIONS (Method 300.0) As AsAs Element CaCo3 As Ion CaCo3 Calcium (Ca) 9 22.5 Chloride (CI 7.9 11.1Magnesium (Mg) 4.4 18.1 Nitrate/Nitrite 1.2 4.3 Sodium (Na) 13.9 30.3Sulfate (SO4) 4 4.2 Potassium(K) 3.7 0.1 Bi-carbonate 55.3 45.3 Barium(Ba) 0.3944 Fluoride (F) 0.2 0.50 Iron (Fe) ND Silica (SiO₂) 53.1Strontium (Sr) 0.1 0.1 Manganese (Mn) ND Copper (Cu) 0.116 Zinc (Zn)0.11 Mg/L GPG Mg/L GPG Mg/L GPG Cations 75.7 4.42 Anions 69.7 4.08Hardness 40 2.4 Additional (CaCO3) Aluminum by ICP N.D. μg/L

TABLE 2 FEED WATER ANALYSIS Inorganics Analyte Method Result Units PQLDilution DLR Antimony (Sb) EPA200.8 ND μg/L 2 1 2 Arsenic (As) EPA200.82.0 μg/L 2 1 2 Barium (Ba) EPA200.8 34   μg/L 5 1 5 Beryllium (Be)EPA200.8 ND μg/L 1 1 1 Cadmium (Cd) EPA200.8 ND μg/L 1 1 1 Chromium -EPA200.8 1.0 μg/L 1 1 1 Total (Cr) Cobalt (Co) EPA200.8 ND μg/L 50 1 50Copper (Cu) EPA200.8 ND μg/L 5 1 5 Lead (Pb) EPA200.8 ND μg/L 5 1 5Mercury (Hg) EPA200.8 ND μg/L 10 1 10 Nickel (Ni) EPA200.8 ND μg/L 10 110 Nitrate (NO3) EPA200.8 4.0 mg/L 1 1 1 Selenium (Se)- EPA200.8 ND μg/L2 1 2 (Total) Silver (Ag) EPA200.8 ND μg/L 10 1 10 Thallium (Tl)EPA200.8 ND μg/L 1 1 1 Vanadium (V) EPA200.8 ND μg/L 10 1 10 Zinc (Zn)EPA200.8 68   μg/L 50 1 50μg/L = micrograms/liter (ppb)PQL: Practical Quantitation LimiDLR: Detection Limit for ReportingPQL × DilutionND: None Detected at DLR

TABLE 3 POST RO TREATMENT Ppm Ppm Ion CaCO₃ Cations Aluminum 0.21 1.18Barium BDL NA Cadmium BDL NA Calcium 0.19 0.5  Chromium(+3) BDL NACopper BDL NA Iron BDL NA Lead BDL NA Magnesium 0.05 0.21 Manganese 0.080.15 Nickel BDL NA Potassium 0.48 0.61 Sodium 0.16 0.35 Zinc BDL NATOTAL 2.96 CATIONS Strong Anions Chloride 02.1  0.30 Chromium(+6) BDL NAFluoride BDL NA Hydroxide BDL NA Nitrate 11.2  9.07 Phosphate BDL NASulfate BDL NA SUB.TOTAL 9.37 Weak Anions Bicarbonate BDL NA CarbonateBDL NA Cyanide BDL NA SUB-TOTAL 0.00 TOTAL 9.37 ANIONS pH 4.21 Units (bymeter) Silica BDL mg/L SiO₂ TOC BDL mg/L COD NA mg/L Carbon NA Ml/gramIsotherm Conductivity 54.0  μmhos/cm Color clear Process Information:System Flowrate (gpm) 6 Operating Temp. (° F.) 120 Hours Operated/Day 16Days Operated/Week 5 Cleaner/System Type aqueous Water Source DI WaterQuality 2 MΩ-cm (Required water quality - 50 Kohms-cm)

Besides providing improved quality conversion coated aluminum products,the process incorporating features of the invention has also been foundto generate less waste products, and the waste products generated areeasier to handle.

Typical waste streams generated by the conversion coating process and DIrecycling procedure are:

-   -   1) depleted coating/processing tank contents,    -   2) processing tank sludge (solids or slurry deposited in the        bottom of processing tanks),    -   3) materials filtered from the DI water, and    -   4) discharge generated by reactivating the deionizing filters.

By employing the DI reprocessing described herein, it has beendiscovered that

-   -   1) The processing tank contents have an extended processing        life,    -   2) The amount of sludge generated is decreased, and can be        readily and safely collected, dried and disposed of as a safe        solid waste, and    -   3) Ionic materials and organic materials discharged from the        filtration system can be processed in a safe manner and the        particulate filters can be dried and discarded as safe solid        waste.

For comparison purposes, set forth below are four examples of proceduresevaluated for applying a non-chromate conversion coating. Theseexamples, while not using applicant's washing and DI water cleaning andrecycling procedure, were less effective experimental attempts to obtaina suitable end product. As a result they all suffer from operatingdeficiencies including varying amounts of streaking, spotting, peelingand discoloration to the finished (treated) product and limitations onbath operating life. Typical prior baths and treatment procedures wasfound to be capable of processing only about 60,000 ft² of acceptableproduct before the treatment baths had to be discarded and new solutionsprepared.

EXAMPLE 1

A four (4) stage system for cleaning and conversation coating aluminumalloy was utilized comprising:

-   -   Stage 1—Sanchem 560 Mild acid cleaner @ 150° F. for 10 min    -   Stage 2—Hot D.I. Rinse @ 140° F. for 30-60 sec.    -   Stage 3—Sanchem CC3400 conversion coating @ 160° F. for 3 min    -   Stage 4—Hot D.I. Rinse @ 140° F. for 30-60 sec.

All process equipment, (hooks, baskets to tanks) was stainless steel.Appearance of the processed aluminum parts was poor, coverage wassplotchy and streaky, film build was irregular and was found to rub off.The process was not adequate to remove oils and dirt on the surfaces.

EXAMPLE 2

An alkaline wash was added to the front end of the process to betterremove oils and dirt. The system includes six (6) process steps.

-   -   Stage 1—Sanchem 8104 silicated alkaline cleaner @ 120-130° F.        for 10 min.    -   Stage 2—D.I. Rinse @ 120-130° F. for 45-60 sec.    -   Stage 3—Sanchem 560 Mild acid cleaner @ 150° F. for 1-2 min.    -   Stage 4—Hot D.I. Rinse @ 140° F. for 45-60 sec.    -   Stage 5—Sanchem CC3400 conversion coating @ 160° F. for 3.0-3.5        min.    -   Stage 6—Hot D.I. Rinse @ 140° F. for 45-60 sec.

The processed aluminum alloy parts were slightly improved in appearancefor a short period of time (about 3 hours), but the life of bathsolutions was not adequate (about 5 hours) due to poor product surfacequality. Coating voids occurred at hang points on work pieces;streakiness and rub off still existed.

EXAMPLE 3

The make-up of stage 3 solution was changed in an attempt to improvecleaning.

-   -   Stage 1—Sanchem 8104 silicated alkaline cleaner @ 120-130° F.        for 10 min.    -   Stage 2—D.I. Rinse @ 120-130° F. for 45-60 sec.    -   Stage 3—Sanchem 6500 acid cleaner @ 150° F. for 1-2 min    -   Stage 4—Hot D.I. Rinse @ 140° F. for 45-60 sec.    -   Stage 5—Sanchem CC3400 conversion coating @ 160° F. for 3.0-3.5        min    -   Stage 6—Hot D.I. Rinse @ 140° F. for 45-60 sec.

The Sanchem 6500 solution contained approximately 3% nitric acid inaddition to CC6500 surfactant. This was an improvement over the processof Example 2. However the appearance of product was still unacceptable.The stainless hangers and baskets were reacting at a different rate thanthe aluminum alloys thereby causing voids at contact points. Oil wasstill not adequately removed.

EXAMPLE 4

The silicated alkaline cleaner (stage 1) was changed to a non-silicatedalkaline cleaner. All baskets and racks were changed from stainlesssteel to aluminum. The steps were as follows:

-   -   Stage 1—Specialty 740 non-silicated, non-caustic alkaline        cleaner @ 150° F. for 3-7 min.    -   Stage 2—D.I. Rinse @ ambient for 45-60 sec.    -   Stage 3—De-Ox 30% Nitric Acid w/Potassium Fluoride @ ambient for        15-60 sec.    -   Stage 4—Spray rinse w/D.I. @ ambient for 45-60 sec.    -   Stage 5—D.I. Rinse @ ambient for 45-60 sec.    -   Stage 6—Sanchem CC3400 conversion coating @ 160° F. for 2.5-3.0        min.    -   Stage 7—D.I. Rinse @ ambient for 45-60 sec.

These changes resulted in a significant improvement in the quality ofthe surface of conversion coated products over the methods, of Example1-3. The products were consistently finishing without the splotchy,streaky appearance and the coating was no longer thickening excessively,thereby eliminating flaking and rub off of the surface coating.Switching to aluminum hangers and baskets also eliminated witness marks.However, the bath life of the chemical solutions was not acceptable.After about 72 hours (or processing about 76,000 ft² of products)streaking re-appeared and filtration cost increased significantly. Thesystem baths had to be changed ever 120 hours (or after processing only127,000 ft² of aluminum parts) as product quality was no longeracceptable.

The filtration system was then modified to increase its ability tohandle the ionic loading, pump sizes were increased to improve flowthrough the recycle system, the efficiency and filtering capacity of thefilters were increased, inert liquid contacting surfaces (tanks,hangers, baskets, etc.) were installed and instrumentation was installedto monitor and control the process, it was discovered that the overallquality of the coated products was significantly increased, by thesechanges, the amount of treatment materials required was reduced and thecost of operating the conversion process was reduced. By improving thefiltration as set forth herein, monitoring the flow rate and maintainingresistance of the wash solution at equal to or greater than about 50K-ohms, it was discovered that the bath life could be increased to 256hours, allowing processing of about 270,352 ft² of aluminum beforeproduct quality started to degrade. The wash solutions could still beused for an additional period, although with decreasing product quality,and had to be discarded and the treating system baths cleaned out ataround 480 hours of processing time. Also, wash water flow rates wereincreased to 10 gpm without degrading the quality of the end product. Itwas also discovered, because the water processing reduced the corrosivenature of the wash solution, that CPVC plumbing materials were adequatefor the purpose of transporting process water through the filtrationsystem. However, PVDF or polypropylene can be used.

De-ionized water is naturally corrosive. While heating the rinses wasexpected to improve the process, because an elevated temperature wouldallow the parts to dry more rapidly, it was found that componentsretained water in cupped areas, and the remaining water would remove thefinish within a couple of minutes. Accordingly, it was also unexpectedlydiscovered that the elimination of heated rinses significantly improvedthe product produced. This improvement was not offset by a slightincrease in drying times which were required in the end stages of theprocess.

It is evident from the foregoing that there are many additionalembodiments of the present invention which, while not expresslydescribed herein, are within the scope of this invention and may suggestthemselves to one of ordinary skill in the art. For example, theinvention may be further improved by adding an additional chemical bathincluding a sealing rinse to add further corrosion resistance andimprove paint and powder coat adhesion or apply, inline, a functionalcoating to be used in place of paint or powder coating. Further, it isnot intended that processing of aluminum incorporating features of theinvention be limited to the chemical compounds specifically identifiedby trade names. Alternatives or competitive compositions may be used.However, critical to the quality of the end product is the washing stepstaken, the extensive cleaning and filtering of the DI wash solutions,and maintaining the electrical resistance of the wash solutions.

It is therefore intended that the invention be limited solely by theappended claims.

1. An improved process for the conversion coating of aluminum productscomprising the steps of: a) immersing an aluminum product in a solutionfor removal of dirt and oil, followed by b) immersion in an oxidizingsolution, and then followed by c) immersion in a solution of apermanganate based conversion composition, the improvement comprising,following each of step a), b) and c) above, subjecting the aluminumproduct, upon removal from immersion in said solution, to at least onespray wash with deionized water and at least one immersion in deionizedwater, the deionized water being processed or continuously reprocessedthrough a series of filters and ion beds to maintain an electricalresistance of at least about 50 k ohms.
 2. The process of claim 1wherein the aluminum product, after each of steps a), b) and c) isexposed to i. a first spray wash with deionized water, ii. followed byimmersion in deionized water, followed iii. by a second spray wash withdeionized water,
 3. The process of claim 1 wherein the deionized wateris maintained at an electrical resistance of at least about 50 k ohms bypassing it through at least a 0.5 micron filter, a carbon filter, acation bed, an anion bed and a DI mixed bed.
 4. The process of claim 1wherein the deionized water is processed serial through a 0.5μ filter, acarbon filter, a cation bed, two anion beds, a DI mixed bed and a 0.5μfilter.
 5. The process of claim 1, wherein at least about 270,000 ft² ofexposed surface of aluminum parts is processed before streaking,spotting, peeling or discoloration of the surface appears.
 6. Theprocess of claim 1, wherein aluminum parts can be processed for at least480 hours before unacceptable streaking, spotting, peeling ordiscoloration of the surface appears.
 7. An improved method of applyinga conversion coaching to aluminum parts, said aluminum parts beingexposed to treatment baths for a cleaning the parts, b) oxidizing theexposed surface of the parts and c) conversion coating the parts,wherein the parts are spray washed and immersed in deionized waterbetween each treatment bath exposure, the deionized water having anelectrical resistance equal to or greater than about 50 k ohms.
 8. Theimproved method of claim 7 wherein the deionized water is maintained ata resistance of at least about 50 k ohms by continuously recirculatingthe water through at least a 0.5μ filter, a carbon filter, a cation bed,an anion bed and a mixed bed at a flow rate from about 3 gal/min toabout 10 gal/min.